This invention relates to the field of ultrasonics. Within that field, the invention relates to ultrasonic transducers.
The use of ultrasound in both non-invasive medical testing and diagnosis and the non-destructive testing of mechanical assemblies such as engines and bridges is known, as are systems which generate and analyze ultrasonic signals. The basic components of an ultrasonic diagnostic system are an electrical signal generator, a transducer which converts the electrical signal into an ultrasonic signal and which receives the reflected ultrasonic signal, and a processor for analyzing the reflected ultrasonic signal.
In medical ultrasonic testing and diagnosis, the most common piezoelectric material used to fabricate ultrasonic transducers is lead zirconate titanate ("PZT"), which is a ceramic material. PZT is used because it has a relatively high piezoelectric coupling constant. Unfortunately, PZT based materials have a relatively large acoustic impedance (30 Mrayl) as compared to the human body (1.5 Mrayl). This large difference in acoustic impedance makes coupling ultrasonic energy from a PZT material into a human body very inefficient. To improve coupling efficiency, transducers are commonly fabricated with one or more matching layers between the PZT ceramic and the human body to improve the coupling and transmission of ultrasonic energy.
The aim is to obtain a proper broad band impulse response.
Substantial efforts have been made to reduce the acoustic impedance of piezoelectric materials by fabricating a composite material made from PZT ceramic and a low acoustic impedance polymer. FIGS. 1a through 1d illustrate several proposed PZT/composite transducer configurations. Composites with a 1-3 connectivity, shown in FIG. 1a, consist of square or circular PZT posts or fibers embedded periodically or randomly in a polymer matrix. A 2-2 composite, shown in FIG. 1b, consists of alternate slabs of PZT and polymer arranged in a periodic manner. A 3-3 composite, shown in FIG. 1c, is made of three dimensional, interconnected porous PZT impregnated with a polymer matrix. A 0-3 composite, shown in FIG. 1d, consists of piezoelectric material in a powder form dispersed throughout a polymer matrix. In each of these examples, the acoustic impedance is substantially lower than that of single phase PZT ceramic and is dependent on the volume percent of PZT used in the composite.
Another constraint on transducer design is the need to match the electrical impedance of the system's electronics and the transducer array. Linear and phased array transducers which are widely used for real-time ultrasonic imaging in biomedical and non-destructive testing applications consist of a one dimensional array of narrow (.about.100 to .about.600 micrometers wide) transducer elements. The fine geometry of the array elements results in the individual elements having a relatively low capacitance. The long cables used to couple the transducer to the system become a very large capacitive load in comparison with the individual elements and result in a poor electrical impedance match between the system's electronics, which includes the cable, and the array elements. Better matching of the electrical impedance of the transducer elements with the combined electrical impedance of the cable and system electronics is desirable.
The use of a composite material in a linear or phased array improves the acoustic matching of the array but, as the dielectric constant of the composite material is lower than that of the PZT ceramic, it simultaneously worsens the electrical impedance match between the system and the array. As the volume percent of polymer in the transducer's composite material increases, the severity of this problem increases. Both the acoustic and electrical impedance matches must be optimized simultaneously to obtain a compact pulse and a relatively wide band response from the transducer.
A known method for improving electrical impedance matching in transducer arrays fabricated with ceramic materials is to construct a multilayer ceramic transducer as shown in FIG. 2. Transducer 30 is comprised of several layers 31 of PZT. Electrodes 32 are placed between layers 31. These electrodes are coupled to external electrodes 39 and 37 alternatively, the electrode between the first two layer being coupled to external electrode 39, the electrode between the next two layers being coupled to electrode 37, and so on. The small insulating beads 35 keep internal electrodes 32 from contacting the wrong external electrode. This technique of increasing the electrical impedance of a ceramic transducer has been used in the ceramic capacitor industry. An N layer multilayer transducer with the same final thickness as a given baseline transducer will have a capacitance which is N.sup.2 more than the capacitance of the single layer transducer. For example, a two layer multilayer transducer has four times the capacitance of a single layer transducer. Using this type of multilayer configuration makes matching the electrical impedance between the system and transducer easier.
Unfortunately, multilayer ceramic transducer 30's material design is not acoustically optimal, as it has the same poor acoustical match to the human body as does a single layer ceramic transducer.
A transducer which can more closely match the acoustic impedance of the human body and simultaneously closely match the electrical impedance of the system is therefore needed.